“Dear Dr. Kim:
“I’m 40 and my boyfriend is 43. We’ve been together for ten years.
“My partner’s sexual fantasy was to watch me have sex with another man.
“Ever since we met, he’s shared that his fantasy is to watch another man have sex with me. Over the past few years, he’s become preoccupied with trying it in real life.
“At first, I was resistant. I am a one-man woman. But the more he asked, the more I realized how happy it would make him. And I thought it might be exciting. Eventually, I agreed to do it — as his birthday present.
“One evening, we went out for drinks and he invited a mate to join us. They’d already agreed this friend would come home with us. Once there, my boyfriend told me to go to the bedroom and get undressed, and to act like he wasn’t there. Then he sent the guy upstairs to join me. He followed a few minutes later and sat in a chair in the corner of the room.
“I was shy initially, but the other guy was very sexy. Once he started kissing me, I relaxed into it and soon found myself enjoying the experience.
“Afterwards, my partner sent him home and got into bed with me and he was so turned on that it was the best sex we’ve ever had.
“Dr. Kim, the problem is that now I’ve let the genie out of the bottle, I can’t put it back in. I did it once -– and now he’s begging me to do it again, but I don’t want it to become a regular part of our sex life.
What am I to do now?
Dear Slave:
Ten years is a long time for a non-marital relationship to last, and it’s an even longer time to be persuading you to fuck another man while he watches.
The unpleasant little truth about seemingly-innocent perversions like this one — and it is a perversion — is that at some point, things are going to start getting unpleasant: for you. Ollie The Onlooker is going get dissatisfied with watching just straight sex, and he’s going to start getting adventurous, which means that you’re going to get dragged along, more or less unwillingly. I don’t know where it will lead to, but as a guess we’re looking at (in no specific order) bondage, possibly sado-masochism, multiple male participants (i.e. gang bangs), and similar fun stuff (for him).
And by the way: your guy’s buddy is going to want a little action without Ollie in the audience. Take my word for it.
As you yourself said, the genie is out of the bottle, so unless you take charge, your future looks bleak.
The man is a sick fuck (literally). Dump him and get on with your life.
— 
The very idea of sloppy assed seconds is unbearable to think about.
….. and It would be hard to explain the dead guy in the bedroom with all the holes in him.
…which explains the expression, “dead kinky”.
if you’re not comfortable with it you shouldn’t do it to please another person. The incremental descent is far too rapid and people regularly fail to see the descent until they’re in a world of discomfort and wonder how they go there.
JQ